History of the Atom

Aristotle

335

Aristotle didn't believe in the atomic theory. He didn't believe that all the materials on Earth are made of atoms, but that they were all composed of four elements of matter: earth, fire, water, and air. Many people believed Aristotle causing Democritis' idea-that all substances on earth are made of atoms-to be overlooked for almost 2,000 years. Eventually Aristotle's view was proven incorrect, and his teachings are not present in modern views of the atom.

Democritus

460

His theory was that, all matter consists of invisible particles called atoms. Atoms are indestructible. Atoms are solid but invisible. Atoms are homogenous. Atoms differ in size, shape, mass, position, and arrangement. Solids are made of small, pointy atoms. Liquids are made of large, round atoms. Oils are made of very fine, small atoms that can easily slip past each other.

Antoine Lavoisier

Oct 30, 1772

Lavoisier found that mass is conserved in a chemical reaction. The total mass of the products of a chemical reaction is always the same as the total mass of the starting materials consumed in the reaction. His results led to one of the fundamental laws of chemical behavior: the law of conservation of matter, which states that matter is conserved in a chemical reaction. We now understand why matter is conserved. Atoms are neither created nor destroyed in a chemical reaction.

John Dalton

Oct 30, 1803

He proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.

J.J. Thomson

Oct 30, 1897

During a series of experiments J.J Thomson discovered the electron. He is also the creator of the plum pudding model (shown in the picture).

Marie and Pierre Curie

Oct 30, 1898

They studied uranium and thorium and called their spontaneous decay process "radioactivity". She and her husband Pierre also discovered the radioactive elements polonium and radium.

Robert Millikan

Oct 30, 1909

Oil drop experiment determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron.

Ernest Rutherford

Oct 30, 1911

The Rutherford model is described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun. The Rutherford atomic model has been alternatively called the nuclear atom, or the planetary model of the atom.

Max Planck

Oct 30, 1918

He is a German physicist best known for being the originator of the quantum theory of energy for which he was given the Nobel Prize for. His work contributed significantly to the understanding of atomic and subatomic processes.

Erwin Schrodinger (2)

Oct 30, 1926

electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud. Thus, this model introduced the concept of sub-energy levels.

Erwin Schrodinger (1)

Oct 30, 1926

In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron. This model can be portrayed as a nucleus surrounded by an

James Chadwick

Oct 30, 1932

He used alpha particles and discovered a neutral atomic particle with a mass close to a proton.Thus discovering the neutron.

Werner Heisenberg

Oct 30, 1958

Werner Heisenberg contributed to the atomic theory by including quantum mechanics, the branch of mechanics, based on quantum theory, used for interpreting the behavior of elementary particles and atoms.

Henri Becquerel

Oct 30, 1986

While studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very pentrating rays.

Niels Bohr

Oct 30, 1992

He developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons.